Silicon photomultiplier trigger network

ABSTRACT

An apparatus includes a plurality of photosensors. Photon trigger signals produced in response to signals from the sensors are received by a trigger line network that includes segment, intermediate), and master lines. The trigger network is configured to reduce a temporal skew introduced by the trigger line network. Validation logic provides a trigger validation output signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 12/672,074 filed Feb.4, 2010 which is a national filing of PCT application Serial No.PCT/IB2008/053164, filed Aug. 6, 2008, published as WO2009/019660 A2 onFeb. 12, 2009, which claims the benefit of U.S. provisional applicationSer. No. 60/954,623 filed Aug. 8, 2007, which is incorporated herein byreference.

DESCRIPTION

The following relates to photodiodes, and especially to arrays ofGeiger-mode avalanche photodiodes. It finds particular application todetectors used in positron emission tomography (PET) systems, singlephoton emission computed tomography (SPECT) systems, optical imagingdevices, high energy physics, and other applications that measure thearrival times of incoming photons.

Various applications in the medical and other domains rely on thedetection of low level light pulses. PET systems, for example, includeradiation sensitive detectors disposed about an examination region,typically in a generally annular or ring-shaped arrangement. Thedetectors, which detect temporally coincident 511 kilo electron volt(keV) gamma photons indicative of positron decays occurring in anexamination region, include a scintillator that generates bursts oflower energy photons (typically in or near the visible light range) inresponse to received 511 keV gammas. Each burst typically includes onthe order of several hundreds to thousands of photons spread over a timeperiod on the order of a few tens to hundreds of nanoseconds (ns).

In a time of flight (TOF) PET system, the relative arrival times of thecoincident gamma photons are used to estimate the position of a positrondecay along a line of response (LOR). For a given activity level andimaging time, the additional TOF data can ordinarily be used to improvethe quality of the reconstructed images.

Photomultiplier tubes (PMTs) have conventionally been used to detect thephotons produced by the scintillator. However, PMTs are relativelybulky, vacuum tube based devices that are not especially well-suited toapplications requiring high spatial resolution. More recently, siliconphotomultipliers (SiPMs) have been introduced. SIPMs have included anarray of detector pixels arranged in a plurality of rows and columns,with each pixel typically including on the order of several thousandavalanche photodiode (APD) cells. Each APD cell includes a photontrigger circuit that produces trigger output signals in response tophotons detected by the cell. The trigger outputs are provided to a timeto digital converter (TDC) that generates digital time stamp dataindicative of the gamma arrival time.

SiPMs can offer a number of advantages, including relatively compactsize, good sensitivity, and good spatial resolution. Moreover, APDs andtheir associated quenching and recharging, triggering, TDC, photonenergy measurement, and other associated circuitry can often befabricated on the same semiconductor substrate. See PCT publication WO2006/111883A2 dated Oct. 26, 2006 and entitled Digital SiliconPhotomultiplier for TOF-PET.

As noted above, the trigger signals from the various APD cells have beenused to trigger the TDC. A conventional trigger line architecture isillustrated in FIG. 1, it being understood that the APDs, quenching andrecharge circuitry, readout logic, and other features of the variouscells and the SiPM have been omitted from FIG. 1 for clarity ofexplanation.

As illustrated, each cell in the array uses an N-type field effecttransistor (NFET) 102 to produce a trigger output signal. The NFETs 102of the cells in each column of the array are connected to a commontrigger line (CTL) 104 in a wired-negative-or (wired-NOR) arrangement,and the CTLs of the various columns are directly connected in parallelto a line 106 that runs along the edge of the APD array. The line 106 isin turn connected to the input of an on-chip TDC 108. As, in the examplecase of a PET system, the accuracy of the position estimate depends onthe accuracy of the timestamp data produced by the TDC, it is generallydesirable to improve the accuracy and consistency of the triggering andresultant time to digital conversion process.

Moreover, APDs can produce output signals even in the absence of adetected photon. As a consequence, SiPMs have also included validationcircuits that suppress trigger signals resulting from dark counts or,stated conversely, accept those trigger signals indicative of detectedphotons. The validation has been performed in parallel with the time todigital conversion process. According to such an arrangement, the TDChas been triggered in response to an initial photon of a scintillationburst. Following (e.g., on the order of about 5 nanoseconds (ns) after)the triggering of the TDC, the output of the validation circuit has beenchecked. If the count is believed to be a dark count, the APD array,trigger lines and the TDC have been recharged and reset, respectively.In a variation, the TDC has been triggered if a given number N ofphotons are detected in a validation time period.

While triggering on an initial photon ordinarily produces a relativelyaccurate timing measurement, the time needed to complete the validationand/or reset process increases the detector dead time, thus tending tolimit the maximum detector count rate. The situation is exacerbated inthe case of an APD that is prone to dark counts. While N-photontriggering can reduce the impact of APD dark counts, the timingmeasurement is typically less accurate.

Aspects of the present application address these matters and others.

In accordance with one aspect, an apparatus includes a semiconductorsubstrate, a plurality of avalanche photodiodes fabricated on thesubstrate, a plurality of photon trigger circuits fabricated on thesubstrate and in operative electrical communication with the avalanchephotodiodes so as to produce trigger signals, and trigger linesfabricated on the substrate. The trigger lines are arranged in ahierarchical structure that includes a first level and a second level.The first level includes a first trigger line that receives triggersignals from a plurality of the trigger circuits. The apparatus alsoincludes a first repeater that receives trigger signals from the firsttrigger line of the first level. The second level includes a firsttrigger line that receives trigger signals from the first repeater.

According to another aspect, a method includes using an avalanchephotodiode of a silicon photomultiplier to detect photons, generatingphoton trigger signals in response to signals from the avalanchephotodiode, receiving the photon trigger signals at a first triggerline, and receiving, via a first repeater of the siliconphotomultiplier, trigger signals from the first trigger line at a secondtrigger line.

According to another aspect, an apparatus includes a semiconductorsubstrate, a plurality of photosensors fabricated on the substrate, anda first trigger line fabricated on the substrate. The first trigger linereceives photon trigger signals produced in response to signals from afirst plurality of the photosensors. The apparatus also includes asecond trigger line fabricated on the substrate. The second trigger linereceives photon trigger signals produced in response to signals from asecond plurality of the photosensors. The apparatus also includes atleast one of photon trigger validation logic and a time to digitalconverter fabricated on the substrate and in operative electricalcommunication with the first and second trigger lines.

According to another aspect, a method of using an apparatus thatincludes an array of photosensors fabricated on a semiconductorsubstrate, a first trigger line fabricated on the substrate, and asecond trigger line fabricated on the substrate is provided. The firstand second trigger lines each receive photon trigger signals generatedin response to signals from a different subset of the photosensors, andthe method includes receiving a photon trigger signal at the firsttrigger line and using circuitry fabricated on the substrate to performat least one of (i) determining if a photon trigger signal was receivedby the first trigger line, determining if a photon was received by thesecond trigger line, and using a result of the first and seconddetermination steps to produce a trigger valid signal; and (ii)performing a time to digital conversion as a function of a triggersignal received by a third trigger line of the substrate, where in thethird trigger line receives trigger signals from the first and secondtrigger lines.

According to another aspect, a method includes generating photon triggersignals in response to signals from avalanche photodiodes of anavalanche photodiode array, validating the photon trigger signalsaccording to a validation criterion, changing the validation criterion,and repeating the steps of generating and validating.

According to another aspect, an apparatus includes an avalanchephotodiode array including first, second and third array portions. Thefirst and second portions are relatively more prone to crosstalk thanthe first and third portions. The apparatus also includes a signalvalidator that validates signals from the array. The signals includevalid signals resulting from photons detected by photodiodes of thearray and signals resulting from crosstalk. The validator uses signalsfrom the first and third portions to produce a first group validationsignal and a signal from the second portion to produce a second groupvalidation signal. The arrangement reduces an effect of crosstalkbetween the first and second array portions.

Still further aspects of the present invention will be appreciated bythose of ordinary skill in the art upon reading and understanding thefollowing detailed description.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 depicts a prior art trigger line architecture.

FIG. 2 depicts an imaging system.

FIG. 3A depicts left portion of hierarchical trigger line architecture.

FIG. 3B depicts right portion of hierarchical trigger line architecture.

FIG. 4 depicts trigger lines of a silicon photomultiplier.

FIG. 5 depicts a butterfly structure.

FIG. 6 depicts a validation circuit.

FIG. 7 depicts a method.

With reference to FIG. 2, a example imaging system 200 includes one ormore radiation sensitive detectors 202, a data acquisition system 203,an image generator 204, and an operator interface 206.

The radiation sensitive detector 202 includes one or more SiPMs 208_(1-y) that produce output data indicative of the energy, arrival times,locations, and/or other characteristics of the radiation received by thedetector. Wavelength shifters 210 such as scintillators may be providedto shift the wavelength(s) of the incoming radiation to more closelymatch the sensitive wavelength(s) or the SiPMs 208, for example in thecase of a PET system. As will be described in greater detail below, theSiPMs 208 include a low temporal skew photon trigger network 212, aconfigurable photon validation circuit 214, and a TDC 216 in operativeelectrical communication with the trigger network 212 and the validationcircuit 214.

Signals from the detectors 202 are received by a data acquisition system203, which produces data indicative of the detected radiation. Again inthe example case of a PET system, the data acquisition system 204produces projection data indicative of temporally coincident photonsreceived by the various SiPMs. Where the system includes time of flightcapabilities, a time of flight determiner uses relative arrival times ofcoincident 511 KeV gamma received by the various SiPMs 208 so as toproduce time of flight data.

An image generator 204 uses the data from the acquisition system 204 toproduce image(s) or other data indicative of the detected radiation.Again in the example of a PET system, the image generator 204 includesan iterative or other reconstructor that reconstructs the projectiondata to form volumetric or image space data.

The user interacts with the system 200 via the operator interface 206,for example to control the operation of the system 200, view orotherwise manipulate the data from the image generator 204, configurethe configurable validation circuit 214, or the like.

Returning momentarily to FIG. 1, the spatial distance and hence thelength of the line between the various cells and the TDC 108 varies as afunction of the position of the cells in the array. According to adistributed resistance-capacitance (RC) model, the propagation delayalong a wire line increases quadratically with the line length:t=k·l ²  Equation 1where t is the propagation delay, k incorporates the distributedresistance and capacitance, and l is the length of the line.

As a consequence, trigger signals generated by cells at differentlocations in the array experience different propagation delays. Forexample, a trigger signal generated by a cell 110 located relativelyfarther from the TDC 108 will exhibit a longer propagation delay thanwill a trigger signal generated by a cell 112 located nearer to the TDC108. The difference in propagation delay time introduces a temporal skewin the measured photon arrival times. The skew, which increases with thesize of the array, compromises the accuracy of the photon arrival timemeasurement.

FIGS. 3A and 3B illustrate a photon trigger network of a typicaldetector pixel of the SiPM 208, it being understood that the SiPM 208ordinarily includes a plurality of similarly configured detector pixels.As illustrated, the trigger network includes a hierarchical,tree-structured trigger line architecture in which the trigger lines areorganized in three (3) levels: lower level or segment trigger lines(STLs) 302, intermediate trigger lines (ITLs) 304, and higher level ormaster trigger lines (MTLs) 306.

The SiPM 208, the components of which are fabricated on a semiconductorsubstrate 301, includes a plurality of APD cells 308. The cells 308,which generate output signals in response to detected photons, includecircuitry such as an APD 310 biased in the Geiger mode, readoutcircuitry 316, quenching and reset circuitry 314, and a photon triggercircuit 312 that includes an NFET or other active semiconductor devicethat is connected to a segment trigger line 302 in a wired-NOR or othersuitable arrangement.

It will be understood by those of ordinary skill in the art that thecells 308 typically include various data and other inputs and outputs,and the SiPM includes various photon counting, energy measurement, andother circuits that have been omitted from FIG. 2 for clarity ofexplanation. Examples of these and other features of the cells 308 andthe SiPM 208 are more fully described in above-noted PCT publication PCT2006/111883A2 and in U.S. Patent Application Ser. No. 60/945,998, filedon Jun. 25, 2007 and entitled Photodiode Self-Test, which publicationand application are expressly incorporated by reference herein in theirentireties.

With continuing reference to FIGS. 3A and 3B, segment trigger lines 302receive trigger signals from one or more APD cells 308. Intermediatetrigger lines 304 receive trigger signals from one or more segmenttrigger lines via repeater(s) 318, and the master trigger line 306receives trigger signals from one or more intermediate trigger lines 304via repeater(s) 320. The master trigger line 306 is used to trigger aTDC 322, either directly or through validation or other logic 324, theconnection and operation of which will be described in further detailbelow.

The various trigger lines 302, 304, 306 are connected to suitablepre-charger or pull-up circuits 326 as indicated generally in FIGS. 3Aand 3B for an example trigger line at each level of the hierarchy, itbeing understood that each trigger line of the trigger line network isordinarily connected to a separate pre-charger or pull up circuit 326.The repeaters 318, 320 preferably include one or more NFETs or othersemiconductor switches. In one implementation, the NFETs are configuredin an open drain configuration, with their drains connected to a higherlevel trigger line in a wired-NOR arrangement. Depending on the numberand nature of the signals received at their respective inputs, therepeaters 318, 320 may also include inverters or other logic, examplesof which will be discussed in greater detail below.

For a given cell 308-TDC 322 distance, the insertion of one or morerepeaters 318, 320 between a cell 308 and the TDC 322 tends to reducethe length of the individual trigger lines 202, 204, 206 compared to thearrangement of FIG. 1, with the degree of reduction depending on factorssuch as the number and placement of the repeaters 318, 320, the routingof the various lines 302, 304, 306, their resistance and capacitance,and the like. In addition to reducing the overall propagation delay inthe various lines (subject to delays introduced by the repeaters 318,320), variations in propagation delay may also be reduced, especiallywhen considered in light of the l² delay dependence posited by thedistributed RC model of Equation 1. Moreover, electrically and/orphysically symmetrical trigger line 302, 304, 306 arrangements may beused to provide similar propagation delays along trigger lines at one ormore levels of the hierarchy.

Turning now to FIG. 4, an example trigger line architecture will bedescribed in relation to an SiPM pixel that includes a plurality of APDcells arranged in array having a boundary or perimeter 400. It isassumed for the purposes of the present example that the cells of thearray are arranged generally in N rows 414 _(1-N) and M columns 416_(1-M). While trigger lines 302, 304 are shown for two (2) examplecolumns 416 _(m) and 416 _(m+1) are shown, the trigger lines of theother columns are similarly arranged.

For the purposes of the trigger line arrangement, each column 416 isdivided into K segments 418 _(1, 2, 3 . . . K), with each segmentincluding a segment trigger line 302. The segment trigger lines 302 areconnected via repeaters 318 to an intermediate trigger line 304 thatruns between the columns 416 _(m), 416 _(m+1). The intermediate triggerline 304 is in turn connected via a repeater 320 to a master triggerline 306.

As illustrated in FIG. 4, the master trigger line 306 runs outside theperimeter 400 and generally along an edge of the array. Alternatively,the master trigger line 306 may be located at the center of or otherwisebetween desired rows 414 of the array. Such an arrangement exploits thephysical symmetry of the array structure and hence further reduces thevariation in propagation delay introduced by the intermediate triggerline 304.

With additional reference to FIG. 5, segment trigger lines 302 ofadjacent columns and segments form substantially spatially symmetricbutterfly structures 502. In the illustrated embodiment, eachintermediate trigger line 304 is connected to K/2 butterfly structures502.

As illustrated, a butterfly structure 502 includes four (4) segmenttrigger lines 302 ₁₋₄. The trigger outputs of L=N/K APD cells 308 _(1-L)are connected to each segment trigger line 302 ₁₋₄ in a wired-NORarrangement via NFETs 504. The segment trigger lines 302 are connectedto the inputs of negative and (NAND) gates 512. The NAND gate 512outputs are in turn connected to the gates of NFETs 504, the drains ofwhich are connected to an intermediate trigger line 304 in a wired-NORarrangement.

As can be seen, the segment trigger lines 302 ₁₋₄ and the connectedcells 308 are substantially spatially symmetric with respect to therepeater 318 and the intermediate trigger line 304, and the NFETS 504are connected to the intermediate trigger line 304 at substantially thesame location. As will be appreciated, the propagation delay along thelength of the segment trigger lines 302 will be substantially similar;trigger signals generated by cells 308 connected to the butterflystructure 502 will likewise tend to have substantially similarpropagation delays when measured by the TDC 322.

Other repeater 318 arrangements are also contemplated. For example, thevarious segment trigger lines 302 may be connected to a single, four (4)input NAND gate, in which case one of the NFETs 504 may be omitted. Inanother example, the various segment trigger lines 302 may connected tothe intermediate trigger line 304 through one or more NOR gates, inwhich case the NFET(s) 504 may be omitted. In yet another example, thevarious segment trigger lines 302 may connected to the intermediatetrigger line 304 through separate NFETs 504, with a suitable inverterprovided at the input of the NFET 504 to provide the correct logic.Still other combinations of positive and negative logic may also beprovided.

Arrangements other than butterfly structures are also contemplated. Forexample, the segment trigger lines 302 may be connected to theintermediate trigger lines 304 singly or in a pair-wise fashion. In theformer case, each segment trigger line 302 may be connected to anintermediate trigger line 304 at a desired location, for example througha dedicated repeater 318. In an example of a pair-wise connection, thefirst 302 ₁ and second 302 ₂ segment trigger lines may be connected tothe intermediate trigger line 304 at a first location, the third 302 ₃and fourth 302 ₄ segment trigger lines may be connected to theintermediate trigger line at a second location, and so on.

Turning now to FIG. 6, a validation circuit 602 of the validation logic324 includes a plurality of trigger line groupers 604 and a validator606. Each grouper 604 receives trigger signals from two or moreintermediate trigger lines 304. While FIG. 6 illustrates an example caseof circuit 602 that receives signals from eight intermediate triggerlines 304, it will be understood that the validation circuit 602 wouldordinarily be configured to accommodate the number of intermediatetrigger lines 304 present in a given detector pixel.

Especially where the detector is prone to cross talk, the intermediatetrigger lines 304 provided to each grouper 604 advantageously receivetrigger signals from non-contiguous regions of the APD array (e.g., fromnon-adjacent rows, columns, or other areas). Each grouper 604 produces agroup trigger signal 608 if trigger signals are received at Q or more ofits inputs during a triggering time window (e.g., on the order of about5 ns in the case of a PET system). The validator 606 receives signalsfrom the various groupers 604 and produces a valid photon trigger signal610 if group trigger signals are received from R or more groupers 604within the triggering time window.

Desired values for Q and R may be established based on the number ofintermediate trigger lines 302 received at each grouper, the number ofgroupers 604, and the desired performance characteristics of the pixeland/or the detector. As illustrated in FIG. 6, for example, each grouper604 receives trigger signals from two intermediate trigger lines 304.According to such an implementation, Q may be set to two, in which casethe groupers 604 may be considered as performing a logical and function.Thus, a valid output signal 610 will be produced only if trigger signalsare received at both inputs of at least R of the groupers 604. Again tothe example of a detector that is prone to cross talk, such animplementation reduces the likelihood that a valid output signal 610will be produced as the result of such cross talk. Note that the valueof R may also be used to impart an energy threshold to the validation,with higher values of R tending to increase the energy threshold.

The pixel may be varied between a first photon triggering mode and anR^(th) photon triggering mode by selectively connecting the TDC 322trigger input to the master trigger line 306 or the output of thevalidation circuit 602 in coordination with an operation of the system200, for example via validation logic 324. Additionally or alternately,the triggering mode may be varied by changing the values of one or bothof Q and R during an operation of the SiPM. In one such example, Q maybe set to one, in which case the groupers 604 may be considered asperforming a logical or function. According to such an implementation,varying the value of Q from two to one will cause the validator 606 toproduce a valid output signal 610 if a trigger signal is received at anyR of the input trigger lines 304. If R is set to one, the validationcircuit 602 produces a valid output signal in response to a first photondetected by the APD array, thus providing a first photon triggeringmode. The groupers 604 may also be omitted, in which case a valid outputsignal 610 will be produced if a trigger signal is received at exactly Rintermediate trigger lines 304.

The triggering validation mode may be selected based on a performancecharacteristic of the SiPM 208, an examination protocol being carriedout by the system 200, an input from a human user, a value of atemperature or other environmental parameter, or the like.

In one example, the dark count performance of the SiPMs 208 may vary asa result of variations in the manufacturing process. At the same time,various types of systems 200 may have differing dark count and/or timingrequirements. Accordingly, the SiPMs 208 may be graded or otherwiseselected for use in a appropriate system 200 type. Thus, SiPMs havingcells 308 that are prone to dark counts may be installed in systems 200having relatively less stringent time measurement requirements, in whichcase the validation mode would ordinarily be set at an R^(th) photontriggering mode, where R is greater than one.

As another example, a pixel of a given SiPM 208 may include one or morecells 308 that are prone to dark counts. Hence, the validation mode maybe varied on a pixel-by-pixel basis based on a measured dark countperformance of the pixel. The validation mode may also be varied on agroup-by-group basis, for example by disregarding trigger signals fromthose trigger lines that receive trigger signals from cell(s) 308 thatare prone to dark counts. Where the individual cells 308 can be disabledor inhibited, those cells 308 having an unacceptable performance canalso be disabled. Hence, if a cell or group of cells 308 in a givenpixel exhibit a dark count rate that is greater than a first dark countrate threshold, the validation criteria for the cell or group of cells308 can be increased. Should the performance of the cell or group ofcells 308 exceed a second, relatively higher threshold, the affectedcell or cells 308 can be disabled.

In other example, the performance characteristics of the SiPMs 208 mayvary with variables such as time and/or temperature. Accordingly, theperformance characteristics may be measured or otherwise determinedfollowing the installation of the SiPM in a given system 200, with thevalidation mode adjusted accordingly. Such an operation may beperformed, for example, by a service engineer in connection with theinstallation and commissioning of the system 200 or periodic or otherservice of the system 200. The validation mode may also be adjustedautomatically or semi-automatically following a user-initiatedmeasurement or a confirmation of a proposed adjustment.

In still another example, the performance requirements of the system 200may vary as function of an examination protocol being performed by thesystem. For example, a first examination protocol may emphasize countrate over accuracy of the timing measurement, while a second examinationprotocol may require a relatively low count rate but have a highertiming accuracy requirement. Consequently, the validation mode may beselected based on the determined requirements of the protocol, forexample on an SiPM 208 by SiPM 208, pixel-by-pixel, or group-by-groupbasis.

Operation will now be described with reference to FIG. 7.

One or more photons are detected at step 702.

Photon trigger signal(s) indicative of the detected photon(s) aregenerated at step 704.

At 706, photon trigger signal(s) are received at one more segmenttrigger lines 302.

At 708, photon trigger signal(s) are received at one more intermediatetrigger lines 304 via repeater(s) 318.

At 710, photon trigger signal(s) are received at the master trigger line306 via repeater(s) 320.

Trigger signal validation is performed at 710. Where the SiPMs 208include a configurable triggering validation circuit, the validationlogic is varied according to the selected trigger validation mode.

The TDC is triggered at step 712, and a data acquisition sequence isinitiated. Note that the validation 710 may be performed following orotherwise temporally in parallel with the triggering of the TDC.

Data indicative of the detected photon(s) is acquired at step 714. Wherethe system 200 includes a plurality of SiPMs 208, the photon data isacquired from the various SiPMs and aggregated or processed as desired.In the case of a TOF PET system, for example, data indicative oftemporally coincident photons is used to produce projection data, withthe relative arrival times being used to estimate the location of apositron annihilation along an LOR.

The acquired data is processed at step 716. In the case of an imagingsystem, for example, the acquired data may be used to generate imagedata indicative of the detected radiation.

The processed data is presented in a human perceptible form at step 718.

Variations and alternatives are contemplated. For example, the varioustrigger lines may be organized in structures having less than or greaterthan three levels. In such a case, the trigger validation circuit 602may receive trigger signals from a desired level (or levels) in thehierarchy.

The above discussion has focused on an APD array organized in pluralityof rows and columns. The various rows and/or columns may be irregular,for example where one or more cells 308 are offset by a fraction of theAPD pitch. Moreover, it will be understood that the various columns neednot be vertical and the rows need not be horizontal. Thus, for example,the columns may be horizontal and the rows vertical.

The SiPMs 208 may be used in imaging and non-imaging systems other thanPET systems and with photodetectors other than APDs.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A method of using an apparatus that includes an array of photosensorsfabricated on a semiconductor substrate, a first trigger line fabricatedon the substrate, and a second trigger line fabricated on the substrate,wherein the first and second trigger lines each receive photon triggersignals generated in response to signals from a subset of thephotosensors, the method comprising: receiving a photon trigger signal;using circuitry fabricated on the substrate to perform at least one of:determining if the photon trigger signal was received by the firsttrigger line; determining if the photon was received by the secondtrigger line, and using a result of the first and second determinationsteps to produce a trigger valid signal; and performing a time todigital conversion as a function of a trigger signal received by a thirdtrigger line of the substrate, wherein the third trigger line receivestrigger signals from the first and second trigger lines.
 2. The methodof claim 1 wherein the first and second trigger lines are connected tothe third trigger line in a wired-NOR arrangement.
 3. The method ofclaim 1 wherein the apparatus includes a plurality of trigger lines,wherein each trigger line of the plurality receives photon triggersignals generated in response to signals from a different subset of thephotosensors, the method including: determining if a trigger signal wasreceived by at least Q₁ members of a first subset of the plurality oftrigger lines, where Q₁ is greater than 1; determining if a triggersignal was received by at least Q₂ members of a second subset of theplurality of trigger lines, where Q₂ is greater than
 1. 4. The method ofclaim 1 wherein the apparatus includes a plurality of trigger lines,wherein each trigger line of the plurality receives photon triggersignals generated in response to signals from a different subset of thephotosensors, the method including: determining if a trigger signal wasreceived by at least R of the trigger lines, where R is greater thanequal to
 1. 5. The method of claim 1 wherein the apparatus forms part ofan examination apparatus and the method includes: determining a protocolof an examination to be carried out using the apparatus; using a resultof the determination to vary a logic used to produce the valid triggersignal.
 6. The method of claim 1 including: determining an operatingcharacteristic of a photosensor; using a result of the determination tovary a logic used to produce the valid trigger signal.
 7. A methodcomprising: generating photon trigger signals in response to signalsfrom avalanche photodiodes of an avalanche photodiode array; validatingthe photon trigger signals according to a validation criterion; changingthe validation criterion; and repeating the steps of generating andvalidating.
 8. The method of claim 7 including determining a performancecharacteristic of the photodiode array, wherein changing includeschanging the validation criterion as a function of the determinedperformance characteristic.
 9. The method of claim 8 wherein determiningincludes determining a dark count performance of an avalanche photodiodeof the array.
 10. The method of claim 8 wherein determining includesmeasuring a characteristic of an operating environment of the array. 11.The method of claim 8 wherein the array includes an avalanche photodiodecell (308) and the method includes varying an operation of the cell as afunction of the determined performance characteristic.
 12. The method ofclaim 7 wherein the array includes an avalanche photodiode cell and themethod includes: evaluating a performance characteristic of the cell;disabling an operation of the cell if the evaluated performancecharacteristic satisfies a first criterion; and changing includeschanging the validation criterion if the evaluated performancecharacteristic satisfies a second criterion.
 13. The method of claim 7wherein the array forms part of an examination apparatus and the methodincludes: determining an examination protocol to be carried out usingthe apparatus; adjusting the validation criterion as a function of thedetermined protocol.
 14. An apparatus comprising: an avalanchephotodiode array including first, second and third array portions,wherein the first and second portions are relatively more prone tocrosstalk than the first and third portions; and a signal validator thatvalidates signals from the array, wherein the signals include validsignals resulting from photons detected by photodiodes of the array andsignals resulting from crosstalk, wherein the validator uses signalsfrom the first and third portions to produce a first group validationsignal and a signal from the second portion to produce a second groupvalidation signal, whereby an effect of crosstalk between the first andsecond array portions is reduced.
 15. The apparatus of claim 14 whereinthe signals are photon trigger signals.
 16. The apparatus of claim 14wherein the first and third portions are physically non-contiguous. 17.The apparatus of claim 16 wherein the array includes a plurality ofcolumns and the first, second, and third array portions includerespective first, second, and third columns of the array.
 18. Theapparatus of claim 14 wherein the array includes a fourth array portionthat is non-contiguous with the second array portion and the validatoruses signals from the second and fourth array portions to produce thesecond group validation signal.
 19. The apparatus of claim 14 whereinthe validator uses the first and second group validation signals toproduce a valid output signal.